In standard medical x-rays a sheet of film is placed in contact with one or two phosphor sheets. The x-rays cause the phosphor to fluoresce, thereby exposing the film. With this method it is critical to insure appropriate exposure for the desired film density. For wider exposure latitude computed radiography utilizes a storage phosphor material, as described in U.S. Pat. No. Re. 31,847, reissued Mar. 12, 1985, inventor Luckey. Part of the absorbed x-ray energy in a storage phosphor causes instantaneous fluorescence, but a significant part is stored in the phosphor and is not emitted as light until this type of media is discharged. The media is discharged by scanning with a relatively long wavelength beam of stimulating radiation, such as red or infrared light. The red stimulating light excites the phosphor causing the release of stored energy as short wavelength blue or violet emitted light. The amount of short wavelength emitted light from each pixel area of the phosphor surface is measured and represents the quantity of x-ray exposure, if the red stimulating energy is constant and illuminating only the pixel being read. Most of the red stimulating light diffusely reflects off the phosphor surface and must be prevented from reimpinging elsewhere on the phosphor where it could discharge energy as blue “flare” light from areas other than the pixel being read. To optimize the signal-to-noise ratio of the imaging system, it is desirable to collect as much of the emitted light as possible and to direct it to the photodetector. John C. Boutet disclosed a V-roof mirror collector in U.S. Pat. No. 4,743,759, issued May 10, 1988, that utilized top and bottom 90° roof mirrors to produce a high efficiency low flare collector using two large diameter PMT's. That design relied on unique properties of 90° roof mirrors, the use of narrow top and bottom slots and very close spacing to the phosphor to achieve high efficiency and low flare. Later he disclosed a Split V-roof mirror collector in U.S. Pat. No. 5,105,079, issued Apr. 14, 1992, that reduced the cost of the original design by utilizing only one PMT. That design split the original 90° roof mirrors in half and reflected the resulting 45° mirrors on themselves with a large vertical mirror to benefit from the properties of 90° roof mirrors. John C. Boutet and Michael B. Brandt went on to disclose in U.S. Pat. No. 5,151,592, issued Sep. 29, 1992, the possible use of second-surface reflector-coated blue filters or other means to produce blue mirrors that reflect blue and do not reflect red light to control flare in CR light collectors. One problem they mention about such blue mirrors is that they generally have some blue absorption that reduces blue light collection efficiency.
For achieving high collection efficiency the mirror reflectivity should be as close to 100% as possible. FIG. 1 shows that the emission spectra of the blue stored energy wavelengths from a typical storage phosphor have a 350 nm to 450 nm distribution peaking at 400 nm and the stimulating radiation is typically a narrow laser output that in this case is at 639 nm.
Uncoated aluminum generally provides around 90% reflectivity in the blue emission range. The reflectivity of aluminum mirrors can be significantly improved with coatings if a single wavelength and reflection angle are involved. In CR light collectors however the coating must work for a wavelength range of a 100 nm and a reflection angles range from near 0 degrees to near 90 degrees from normal. By enhancing the aluminum coating with 4 to 6 coating layers the average reflectivity can be enhanced to around 95%. Blue filter mirrors produced by aluminizing the back face of a blue filter will generally yield reflectivities below 90%. To avoid the efficiency losses produced by filter mirrors, collector designs have generally controlled flare by optimizing collector geometry for low flare and high efficiency. This has restricted the design space one can explore for high efficiency.
The width of the plate being scanned and the geometry of the collector determine the space envelope that a collector design requires. The Split V-roof design described in U.S. Pat. No. 5,105,079 requires that the PMT extend out past the side of the phosphor plate thereby increasing the required footprint of most scanner designs utilizing that collector. U.S. Pat. No. 5,134,290, issued Jul. 28, 1992, inventors Boutet et al., and U.S. Pat. No. 5,140,160, issued Aug. 18, 1992, inventors Boutet et al. describe pyramidal mirror collectors which position a single mirror centrally in a collector design that could permit a smaller footprint scanner. All of these collector designs require a large size PMT face in order to achieve good collection efficiency. Such large PMT's are generally expensive. Also the collection efficiency of these designs varied significantly across the width of the phosphor and the width of the collection slot in the page-scan direction had to be narrow in order to keep flare at an acceptable limit.
It is desirable for a collector design to provide a reasonably uniform and smooth collection efficiency profile across the width of the collector so that the profile can be easily and repeatedly corrected out with a look-up table. Sharp discontinuities in a collection profile are likely to shift slightly over time with temperature or other factors and the correction profile, if not updated, will then introduce visible sharp streaks in the resulting image. Michael B. Brandt discloses an efficient light collector with a uniform and smooth profile in U.S. Pat. No. 5,506,417, issued Apr. 9, 1996. This design utilizes five 3″ square-faced PMT's.